Your tablet is ONDA V820W V5 then. Hardware like Wi-Fi/touchscreen/cameras might be different, but it should boot at least. You need to have 100 mb EFI partition and copy GRUB/kernel there, and ext4 root partition, large enough to contain Sailfish OS files.

Are the devices identical hardware-wise? As first step, extract boot paritition from stock firmware. Try to repack it and make sure it can be booted (flash or boot directly via fastboot). Then try replacing kernel with the one found on Dell Venue 8 7840 Android 5.0 firmware and check if it still can boot (kernel modules won't match, but display driver is usually built into kernel).

i don't know if they are identical. thanks for the ideas. why do i need the new kernel? for drivers? btw i broke ground on USB ,so ibrought it to arepair shop

Your tablet is ONDA V820W V5 then. Hardware like Wi-Fi/touchscreen/cameras might be different, but it should boot at least. You need to have 100 mb EFI partition and copy GRUB/kernel there, and ext4 root partition, large enough to contain Sailfish OS files.

Are the devices identical hardware-wise? As first step, extract boot paritition from stock firmware. Try to repack it and make sure it can be booted (flash or boot directly via fastboot). Then try replacing kernel with the one found on Dell Venue 8 7840 Android 5.0 firmware and check if it still can boot (kernel modules won't match, but display driver is usually built into kernel).

or can sb send me the contents of the boot partition?^^ it doesn't hurry though, because the tablet is still at the repair shop

You don't need boot partition. The kernel from /boot folder in rootfs is loaded using GRUB. We can't use Android bootloader, since UEFI is 32-bit and Jolla Tablet has 64-bit kernel (it's possible to compile 32-bit one, but I had some PulseAudio-related system crashes).

You don't need boot partition. The kernel from /boot folder in rootfs is loaded using GRUB. We can't use Android bootloader, since UEFI is 32-bit and Jolla Tablet has 64-bit kernel (it's possible to compile 32-bit one, but I had some PulseAudio-related system crashes).

so i just have to copy the files to a partition and install grub with reference reference to the kernel

One thing though, the tablet of yours is ONDA V820W V5, while mine is V3. There are slight hardware changes, but I'm not sure what is different exactly. The image should boot, but touchscreen/Wi-Fi/Bluetooth/sound/etc may not work depending on what has been changed.